Electroceramic components with a temperature fuse includes varistors, for example The base body of such components is often produced from a mixture of various metal oxides, such as zinc oxide. Varistors have a non-linear voltage-dependent resistance change that is used to protect an electric circuit from overvoltage. The resistance value of a varistor drops as the applied voltage increases.
When an overvoltage appears (e.g., operating voltage that exceeds a permitted limit value for the varistor), current passing through the varistor rises sharply. As a result, the varistor heats-up. A long-lasting overvoltage can lead to overheating and set-off a fire.
A varistor fuse element for protecting an electric circuit from overvoltage and overheating is known from printed document DE 331 85 88. It consists of a mechanical construction in which solder with a low melting point is applied to the varistor, which establishes a spring current conductor. When an overvoltage and associated overheating appears, the solder melts, whereupon the spring structure immediately produces an irreversible low-resistance short circuit with a second current conductor. In this case, the spring structure also increases the distance between the spring current conductor and the varistor, in order to prevent an electric flashover. A disadvantage of this mechanical structure is in the fact that it can be achieved only at great cost.
In printed document JP 04 151 804 A, a temperature fuse is disclosed, that is integrated into a varistor housing, and that is connected through an electric line to an internal electrode of the varistor. The temperature fuse, in this case, is surrounded by a material that permits heat to be conducted between the varistor and the temperature fuse. In case of overheating of the varistor due to long-lasting overvoltage, the heat of the varistor can thereby be transferred to the temperature fuse and trigger it. The disadvantage of this structure is that the temperature fuse is not in direct thermal contact with the varistor. Therefore, because of losses during heat transfer, the fuse is triggered only at higher temperatures.